do I still need a theme?
Yes! You always need a theme in WP, Elementor is still just a plugin, it lets you put various custom content, but for Elementor specifics you should contact elementor support, 3rd party plugins are offtopic on this stack.
Keep in mind that without a theme, there’s no main post loop, and without a main post loop there’s no way to show post content. As a result you could produce the most magnificant layout in Elementor or another page builder plugin, and there’d be no mechanism by which to show it.
can I uninstall the default themes?
Yes, the default themes are just prebundled.
is there a super light theme just for typography, given that I won’t use it for anything else?
Recommendations should go on the software recommendations stack, but there are lots of options here. The absolute minimum theme WP needs contains a style.css and an index.php
I assume a “theme” is similar in concept to a ui framework (e.g. bootstrap / material design), with some typography, css presets, js widgets, etc.
Yes and no, a Theme provides at the minimum the fallback template index.php from the template hierarchy, and a style.css that provides. the theme name/description in a comment at the top.
Themes don’t have to provide a UI framework, JS, or anything at all. Theme templates are what get shown on the frontend. Without a theme, WP will call wp_die complaining that there’s no theme, or, you’ll get a blank page with no markup.
Themes also provide the basic HTML structure of a page, and the appropriate places to call the header/footer hooks, page title, etc
There’s also a functions.php that if present gets loaded before the template is loaded, and gives you a chance to add filters and hooks, create objects, etc.
If you’re looking to build out the entire page using only Elementor, or have Elementor questions, you should contact Elementor support, check Elementor docs, or ask in Elementor communities. This stack is for WP development, but your questions require specialist knowledge in Elementor that only Elementor communities/docs/support can provide.